Image sensors are widely used in digital still cameras, cellular phones, security cameras, as well as in, medical, automobile, and other applications. Complementary metal-oxide-semiconductor (“CMOS”) technology is used to manufacture lower cost image sensors on silicon substrates. In a large number of image sensors, the image sensor commonly includes hundreds, thousand or even millions of light sensor cells or pixels. A typical individual pixel includes a micro-lens, a filter, a photosensitive element, a floating diffusion region, and one or more transistors for reading out a signal from the photosensitive element. One of the transistors included in the typical pixel is commonly referred to as a transfer transistor, which includes a transfer gate disposed between the photosensitive element and the floating diffusion. The transfer gate is disposed on a gate oxide. The photosensitive element, floating diffusion region, and gate oxide are disposed on a substrate.
During operation of a typical pixel, a conducting channel region may be formed under the transfer gate when a bias voltage is applied to the transfer gate such that an image charge is transferred from the photosensitive element to the floating diffusion region. However, conventional pixels often suffer from image lag, blooming, and manufacturing challenges.
Image lag may result from the conventional transfer transistor being unable to remove all the signal from the photosensitive element such that a residual signal remains during successive readings of the pixel. This leftover information remaining in the photosensitive element is often referred to as image lag, residual image, ghosting or frame-to-frame retention.
Blooming may result from high intensity portions of an image which cause photo-generated excess charge carriers to spill into adjacent photosensitive elements. In one design of a transfer transistor, an N-doped polysilicon gate electrode controls a surface channel transistor as in a standard NMOS transistor. In this design, the transistor's threshold voltage is low and a negative gate bias generally needs to be applied during an integration period and a large gate voltage swing is required to minimize image lag. In this case, blooming may result and blooming may limit the imaging sensor's dynamic range and may limit the types of commercial applications of the imaging sensor.
Manufacturing challenges in the conventional pixel may stem from the position-sensitive placement of the photosensitive element in relation to the transfer gate. The sensitive nature of the placement may result in increased defects in parts and increased manufacturing costs.